The use of dimming apparatus in electric lighting, including gas discharge lamps, is well-known. Examples include two patents to Black, Jr., et al. (U.S. Pat. Nos. 4,947,079 and 4,899,088) which disclose circuits for providing dimming of fluorescent lights by creating "notches" in waveforms supplied by an alternating power source to a conventional inductive ballast. U.S. Pat. No. 4,663,569 to Alley et al. and U.S. Pat. No. 3,935,505 to Spiteri also show fluorescent lamp dimming systems which can be used in a retrofit manner with existing lamp ballasts. U.S. Pat. No. 4,350,935 to Spira and Richardson U.S. Pat. No. 5,194,782 illustrate other approaches to dimming gas discharge lamps.
Other technologies exist to "vary" the output of gas discharge lamps, without providing a full range of conventional dimming functions. Patents such as those to Konopka, et al. (U.S. Pat. Nos. 5,373,218 and 5,194,781) describe control circuits for fluorescent lamps, using a conventional two-position switch and conventional wiring, but only switching between two states--one "high energy" and one "low energy".
Certain other electronic ballasts (ballasts with built-in circuits) already exist to control the brightness of gas discharge lamp(s). These electronic ballasts are more energy efficient than conventional ballasts, and typically operate by changing the operating frequency of the ballast to maintain a current through the ballasts which is controlled by a reference voltage. Since there is typically an inductor in series with the lamp, increasing the frequency reduces the current to the lamp. The brightness of the lamp is therefore ultimately controlled by varying a reference voltage. Such an approach typically requires additional wiring and/or external circuitry, however, to vary the reference voltage in the ballast and thereby provide the necessary signal. Even for new construction, the costs of installation are increased because, among other things, of the need for the additional set of wires between the switch and the lamp(s). This approach also increases the expense, effort and time required for retrofit situations.
As indicated above, the foregoing and other prior art devices have various shortcomings. Certain of them use too much power, and/or are subject to short-circuiting. Many do not provide dimming over a desirably broad range of light output. Some require additional wiring between the switch and the lamp, thereby making them unsuitable for easy retrofits of existing installations.